1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer which use an electrophotographic process or electrostatic recording process. More specifically, the present invention relates to an image forming apparatus that includes an intermediate transfer member and a recording material bearing member as a rotatable belt member disposed adjacent to an image bearing member on whose surface a toner image is formed, and the belt members can abut on and separate from the image bearing member.
2. Description of the Related Art
Conventionally, a color image forming apparatus capable of forming full-color images which uses a direct transfer method or an intermediate transfer method has been known. In the direct transfer method, a toner image formed on a single or a plurality of photosensitive drums which are image bearing members is transferred to a recording material borne on a revolving belt member, (“transfer belt” hereinafter) as a recording material bearing member. In the intermediate transfer method, a toner image formed on a single or a plurality of photosensitive drums is transferred on a rotatable belt member as an intermediate transfer member (“intermediate transfer belt” hereinafter). Then, the toner image on the intermediate transfer belt is transferred to the recording material. The intermediate transfer method can easily form images on various recording materials and enhance recording material selectivity.
Further, a configuration where circumferential speeds of the intermediate transfer belt and the transfer belt are varied relative to the photosensitive drum has been known as an effective method for improving transfer efficiency. Japanese Patent Application Laid-Open No. 2006-267705 discusses a technique to improve the transfer efficiency by weakening an adhesive force of a toner image formed on a photosensitive drum by applying a shearing force thereto caused by rubbing with an intermediate transfer belt.
In the color image forming apparatus, an image of only a black single color may be formed on the photosensitive drum (i.e., one image bearing member). In this case, a problem arises that when an image bearing member other than that for black is driven, the image bearing member and other related members maybe worn. To deal with this problem, Japanese Patent Application Laid-Open Nos. 2001-249519, 2004-117426, and 2005-62642 discuss a configuration where an image bearing member other than that for black separates from a transfer belt or an intermediate transfer belt during formation of only a black image.
FIG. 9 illustrates a configuration of a conventional image forming apparatus which uses the intermediate transfer method. Four process units that are image forming units are provided corresponding to respective colors of yellow, magenta, cyan and black. The image forming apparatus includes photosensitive drums 1a to 1d, charge units 2a to 2d, exposure units 3a to 3d, development units 4a to 4d, an intermediate transfer belt 51, a tension roller 52, primary transfer members 53a to 53d, a drive roller 55, cleaning units 6a to 6d, and secondary transfer units 56 and 57.
When a full-color image is formed, the charge units 2a to 2d uniformly charge the photosensitive drums 1a to 1d, and then the exposure units 3a to 3d perform exposure according to an image signal and electrostatic latent images are formed on the photosensitive drums 1a to 1d. The development units 4a to 4d develop toner images. The toner images on the photosensitive drums 1a to 1d are sequentially transferred to the intermediate transfer belt 51 by applying transfer biases to the primary transfer members 53a to 53d from a high voltage power source for transfer (not shown). In this case, by setting a roller for regulating a position of the intermediate transfer belt at a position 58a (broken line), the intermediate transfer belt abuts on the four-color photosensitive drums (arrangement illustrated with the broken line). The cleaning units 6a to 6d collect the toner remaining on the photosensitive drums 1a to 1d after transfer. The images which are transferred sequentially from the respective photosensitive drums and overlaid to the intermediate transfer belt 51 in the aforementioned manner are transferred to a recording material P by a secondary transfer bias applied between the secondary transfer materials 56 and 57. The toner images transferred to the recording material P are fixed by a fixing unit (not shown) to obtain a full-color image.
When a black single color image is formed, the roller for regulating the position of the intermediate transfer belt is set in a position 58b to separate the photosensitive drums 1a, 1b and 1c for forming yellow, magenta and cyan images. The intermediate transfer belt is accordingly disposed as indicated by a solid line in the drawing. Thus the black single color image is formed on the photosensitive drum 1d, and transferred by the primary transfer member 53d to obtain the single color image. The photosensitive drums 1a, 1b and 1c for other three colors are stopped to prevent wearing.
However, during an imaging operation or imaging preparation where a black toner image is formed, a color mode may switch between a black single color mode and a full-color mode. When the photosensitive drum rotating at a circumferential speed which is greatly different from that of the intermediate transfer belt is separated from the belt member such as the intermediate transfer belt, a belt driving load changes and a shock transmitted to the belt member increases. Consequently, uneven rotation of the belt member caused by the shock affects an image during the imaging operation or the imaging preparation. A similar problem occurs when the intermediate transfer belt abuts on the belt member.
To prevent such a defective image caused by shocks to the belt member, Japanese Patent Application Laid-Open No. 2001-183888 discusses an apparatus which includes a delay circuit for outputting image data after the secondary transfer member abuts and fluctuation in intermediate transfer member speed is sufficiently contained.
However, in the configuration discussed in Japanese Patent Application Laid-Open No. 2001-183888, when the photosensitive drum rotating at the same circumferential speed as that during image formation with respect to the belt member is separated from or abuts on the belt member, time required to absorb the shock is extended. Consequently, time until an imaging start on the photosensitive drum abutting on the belt member is extended, or an image defect occurs during the imaging operation. Thus, there is a demand for a configuration which can reduce a shock to the belt member as much as possible by moving the image bearing members which rotate at different circumferential speeds with respect to the belt member during image formation.